linux-kernel - Re: [PATCH v9] perf: add qcom l2 cache perf events driver

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Date:   Tue, 7 Feb 2017 10:43:15 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     "Leeder, Neil" <nleeder@...eaurora.org>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        Mark Langsdorf <mlangsdo@...hat.com>,
        Mark Salter <msalter@...hat.com>, Jon Masters <jcm@...hat.com>,
        Timur Tabi <timur@...eaurora.org>, cov@...eaurora.org
Subject: Re: [PATCH v9] perf: add qcom l2 cache perf events driver

On Mon, Feb 06, 2017 at 02:11:36PM -0500, Leeder, Neil wrote:
> Hi Mark,
> Thanks for those comments - I'll add the fixes.

Cheers!

> On 2/6/2017 10:48 AM, Mark Rutland wrote:
> >I'm still concerned by this use of the filter_match callback, because it
> >depends on the set of other active events, and can change as other
> >events are scheduled in and out.
> >
> >When we schedule in two conflicting events A and B in order, B will fail
> >its filter match. When we scheduled out A and B in order, B will succeed
> >its filter match.
> >
> >The perf core does not expect this inconsistency, and this appears to
> >break the timing update logic in event_sched_out(), when unconditionally
> >called from ctx_sched_out() as part of perf_rotate_context().
> >
> >I would feel much happier if we dropped l2_cache_filter_match(), at
> >least for the timebeing, and handled this as we do for other cases of
> >intra-pmu resource contention.
> >
> >We can then consider the filter_match addition on its own at a later
> >point.
> 
> So could this be detected in get_event_idx, the same way we handle
> counter resource contention? That would eliminate filter_match, and
> it's the same way its done in armv7
> (arch/arm/kernel/perf_event_v7.c:krait_pmu_get_event_idx()).

Returning -EAGAIN from event_get_ixd() in that case sounds good to me.

Thanks,
Mark.